Modern office lighting has changed primarily because of the advent of VDT (visual display terminals) screens and to some degree because of concerns with direct glare which causes discomfort to an occupant at normal viewing angles. Undirected lighting can cause a reflection and glare on VDT screens. The reflection causes a reduction in the readability of the screen and eye strain results.
Research has shown that if the candlepower distribution of a luminaire is limited to an approximate 55.degree. cone around nadir, reflections on the VDT screens are eliminated and the direct glare is reduced or eliminated as well.
Low brightness louver grids have been developed to control the candlepower distribution curve into an optimal "low direct glare " cone configuration. U.S. Pat. No. 2,971,083 to E. R. Phillips et al. discloses a low brightness louver in which individual intersecting louvers are spaced apart and provided with parabolic surfaces for confining light to a prescribed normal symmetrical candlepower distribution curve. This type of lighting grid has been very successful in reducing the reflection in VDT screens and in elimination of glare. However, a new problem called the "cave effects" has resulted from the use of such grids. The cave effect is the result of no direct light applied to upper portions of walls adjacent to the luminaire because of the severe cut off of light in the luminaire. Thus, an area of low illumination or shadow appears at the upper wall surface adjacent the juncture of the wall and ceiling. Directional lighting gas been used to eliminate the cave effect. However, directional lighting requires additional wattage and the overall efficiency of the lighting system dimishes. Due to the ever increasing need to conserve enrgy, codes and legislation have been introduced to minimize the lighting wattage per square foot of room space, especially in commercial construction.
Kingston Industries Corporation of Liberty, New York has developed a "Kinglux Directional Louver" arrangement for directing light in a manner to substantially reduce or eliminate the cave effect. The louvers are spaced and shaped to concentrate light in a particular direction and therefore utilize wattage only to illuminate the shaded areas otherwise produced by the adjacent low glare parabolic fixtures. Such directional lighting to overcome the cave effect does reduce or eliminate that particualr problem but is not conducive to minimal operating lighting requirements of a structure.
U.S. Pat. No. 4,222,094 to Wolar discloses a light distribution fixture in which a group of nine sets of different angularly oriented louvers are provided to attain varying light distribution characteristics. In variations, several of the louvers are angularly adjustable to provide varied distribution effects. This apparatus does not disclose the use of parabolic louvers and so light emitted through the louver sections is diffused even with the particular angular orientation of the louvers or veins. This results in relatively uncontrolled light distribution and resutling inefficiency. Futhermore, the structure is relatively complex.
A quasi-directional monosymmetrical lighting system is disclosed in the McNamara U.S. Pat. No. 4,042,817 . This patent discloses a directional ceiling mounted fixture which is used to provide indirect bounce lighting from wall structures. Angularly oriented flat louvers are used to directionally orient the light, along with an internal parabolic reflector. The angularly oriented light is arranged within the room to reflect from wall surfaces, however, the angle is such that the light concentration is downward from the juncture of the wall and ceiling. The device is therefore likely to produce the "cave effect" when in operation.
U.S. Pat. No. 4,384,318 to Reibling discloses a task light in which a florescent tube is mounted above spaced, angularly oriented flat louvers. The louvers are angled at approximately 30.degree. to the plane of the light. However, due to the flat orientation of the louvers, diffusion is predominant and control of the candlepower distribution is severely limited.
U.S. Pat. No. 4,621,309 Grawe discloses an elongated luminaire in which a set of parabolic louvers are positioned in a light fixture with upwardly inclined baffles or light guiding plates. The light guiding plates are intended to initially orient the rays of light passing through the grid from an above oriented light. This is to control light distribution through a bidirectional orientation so light is distributed in opposed angular directions from the substantially vertically oriented parabolic louvers. However, diffusion again creates a problem with this arrangement due to the flat nature of the light guiding plates.
Transparent louvers are shown in U.S. Pat. No. 4,368,504 to Sato et al. This patent discloses task lighting using a specially configured refractor member under a light. The refractor is intended to prevent "veiling" reflection on desk or other work surfaces. Again, the louvers are angularly oriented with the intent to control the distribution of light. However, the louvers are flat and diffusion again becomes a problem.
As may be seen from above, the need remains for a light distribution system in which the "cave effect" is effectively eliminated without increasing the wattage requirements for a given room and while maintaining a high lighting efficiency and through which low direct glare light is simultaneously distributed into the adjacent area.